The present invention relates generally to apparatus and methods for measuring downhole conditions within boreholes and more particularly relates to apparatus and methods for monitoring downhole characteristics of the drill string during drilling.
Various measuring-while-drilling techniques for telemetering data representing downhole conditions during drilling of a well have been suggested. U.S. Pat. Nos. 4,100,528 and 4,103,281 and application Ser. No. 727,685, all assigned to the assignee of the present invention and filed Sept. 29, 1976 disclose a measuring-while-drilling system utilizing a phase-shift-keyed modulation system. Logging-while-drilling systems utilizing analog motor control systems in phase-shift-keyed modulation systems are disclosed in U.S. Pat. Nos. 3,309,656; 3,789,355; and 3,820,063. The measuring-while-drilling and logging-while-drilling systems referenced therein disclose telemetry systems for monitoring downhole conditions concurrently with the drilling of the borehole.
The referenced measuring-while-drilling patents disclose various downhole conditions which may be monitored. For example, sensors may be provided for monitoring the direction of the hole, weight on bit, temperature conditions, natural gamma radiation, and formation resistivity.
There are other downhole conditions which are significant during the drilling of a borehole. Since boreholes are rarely straight, knowledge of the crookedness of the borehole is important in that excessively crooked boreholes adversely affect rotation of the drill pipe and adversely affect preproduction and production processes, such as the ease of casing placement and the wear of the sucker rods. U.S. Pat. No. 2,930,137, issued Mar. 29, 1960 to Arps suggests one attempt to solve the borehole crookedness problem using a measuring-while-drilling technique which attempts to detect the beginning of a dogleg by detecting bending moments.
The causes of borehole crookedness or deviation have been analyzed and at least in part have been attributed to the relative flexibility of the drill string and drill collars and to the forces acting on the string that cause it to bend. These forces include forces due to drill string mechanics including weight-on-bit and weight-of-the-drill collar, and forces due to interaction of the drill bit and the rock.
The significance of being able to respond to downhole conditions for controlling deviation of the borehole has been recognized. Proposals for controlling borehole deviation and direction have included control of the weight-on-bit parameter, and the control of drill string flexibility, such as control of the drill string diameter and the use and placement of stabilizers. In connection with these proposals computer analysis techniques have been developed for statically characterizing a given drill string for a given earth condition. The computer analysis provides a proposal, for example a particular placement of stabilizers and a particular weight-on-bit for a given drill string structure, which is to controllably introduce a particular bend in the drill string during drilling. The bend is attempted to be induced in the proper direction to effect the desired borehole deviation and direction.
Despite the various proposals and suggestions for controlling borehole deviation, it is believed that there have been no effective ways yet devised for predicting the future course which a drill bit will take during the drilling of a borehole. Techniques which require analysis of the previously drilled borehole are unsatisfactorily slow and burdensome and perhaps unreliable. Techniques which attempt to controllably bend the pipe during drilling are generally unreliable since there is presently no known way to monitor the nature, i.e., the direction and the degree, of the bend which has been induced. Accordingly, it would be highly desirable to provide a system which yields information on downhole drilling variables which is useful in projecting the anticipated course of the bit during continued drilling, thereby allowing corrections to be effected to the drilling operations for controlling deviations while the drill string remains in the borehole.